In recent years, a laser oscillator for emitting radially polarized laser light attracts attentions. The radially polarized laser light refers to laser light in which an electric field oscillates in a radial direction of a beam spot, as shown in FIG. 4 (a). Meanwhile, laser light in which an electric field oscillates in an identical direction as shown in FIG. 4 (b) is called linearly polarized laser light; laser light in which an electric field oscillates in an azimuth direction of a beam spot as shown in FIG. 4 (c) is called azimuthally polarized laser light; and laser light in which an electric field oscillates in random directions is called randomly polarized laser light. The radially polarized laser light has a smaller spot size at a focal point, as compared with laser light having other polarization states. Therefore, the radially polarized laser light has an advantage of providing high processing efficiency when used in laser processing.
The radially polarized laser light can be generated by using, for example, a microbend fiber grating (see Patent Literature 1 and Non-Patent Literature 1). The microbend fiber grating is an optical element realized by causing a spatially periodic external force to act on an optical fiber from opposite directions alternately with use of two stressors on which recesses and protrusions are provided in a spatially periodic manner. The microbend fiber grating converts, into radially polarized laser light, laser light having a wavelength corresponding to the spatial period of the external force.
FIG. 5 shows a configuration of a conventional laser oscillator 5 which emits radially polarized laser light. The laser oscillator 5 is a laser oscillator including, as a laser cavity, an optical fiber 53 whose both ends are terminated by a mirror 52 and a half mirror 54, respectively. Further, the laser oscillator 5 converts laser light outputted from the laser cavity, into radially polarized laser light with use of a microbend fiber grating 55.
The optical fiber 53, which serves as an amplification medium in the laser oscillator 5, is an active fiber including a core doped with a rare earth element. Upon absorbing pumping light, the rare earth element is transferred to a state of population inversion. When pumping light emitted by a light source 51 enters the optical fiber 53 via an optical fiber 56, laser light is generated by stimulated emission from the rare earth element, which has been transferred to the state of population inversion.
One end of the optical fiber 53 on the light source 51 side is terminated by the mirror 52, which transmits the pumping light emitted by the light source 51 and which reflects, at a certain reflectance, the laser light generated by stimulated emission from the rare earth element. On the other hand, the other end of the optical fiber 53, which end is located on the opposite side to the light source 51, is terminated by the half mirror 54, which reflects, at a certain reflectance, the laser light generated by stimulated emission from the rare earth element and which transmits, at a certain transmittance, the laser light generated by stimulated emission from the rare earth element.
Consequently, the laser light generated by stimulated emission from the rare earth element resonates within the optical fiber 53 and is amplified recursively. Then, part of the laser light amplified recursively within the optical fiber 53 is transmitted through the half mirror 54 and outputted to the outside of the optical fiber 53.
Note that the optical fiber 53 is a single-mode fiber, which is capable of confining a fundamental mode only. Here, the fundamental mode refers to a mode whose light intensity distribution in a cross section of an optical fiber has no node. Typically, in the case of the fundamental mode, the light intensity distribution in the cross section of the optical fiber takes a single-peaked pattern. The fundamental mode is a waveguide mode constituted by two linearly polarized components whose polarization directions are orthogonal to each other. Thus, the laser light, which is transmitted through the half mirror 54 and outputted to the outside of the optical fiber 53, includes the two linearly polarized components mixed together. However, within the optical fiber 53, a phase difference between the two linearly polarized components is not determined to a certain value. Furthermore, within the optical fiber 53, wavelengths of the respective two linearly polarized components can be different from each other. Moreover, within the optical fiber 53, the two linearly polarized components can be coupled to each other. Therefore, the laser light which is transmitted through the half mirror 54 and outputted to the outside of the optical fiber 53 typically becomes a randomly polarized wave.
The fundamental-mode laser light which is transmitted through the half mirror 54 and outputted to the outside of the optical fiber 53 is guided to the microbend fiber grating 55 via an optical fiber 57. The microbend fiber grating 55 converts the fundamental-mode laser light entered, into radially polarized laser light. The radially polarized laser light which is outputted from the microbend fiber grating 55 is guided to an optical fiber 58, and is then outputted to the outside from an end of the optical fiber 58, which end is located on the opposite side to the microbend fiber grating 55.